In recent years, there is an increasing need for the use of electric waves in response to a ubiquitous network society, and a wireless personal area network (WPAN) which realizes wireless broadband at home or a millimeter waveband wireless system, such as a millimeter-wave radar, which supports safe and secure driving starts to be used. An effort to realize a wireless system at a frequency equal to or greater than 100 GHz is actively made.
In regard to second harmonic evaluation of a wireless system in a 60 to 70 GHz band or evaluation of a radio signal in a frequency band equal to or greater than 100 GHz, as the frequency becomes high, the noise level of a measurement device and conversion loss of a mixer increase and frequency precision is lowered. For this reason, a high-sensitivity and high-precision technology of a radio signal over 100 GHz has not been established. In the conventional measurement technologies, it is not possible to separate harmonics of local oscillation from the measurement result, and there is difficulty in strict measurement of unnecessary emission or the like.
In order to overcome the problems in the related art and to realize high-sensitivity and high-precision measurement of a radio signal in a frequency band equal to or greater than 100 GHz, it is necessary to develop a narrowband filter technology of a millimeter waveband for the purpose of suppressing an image response and a high-order harmonic response, and in particular, there is a demand for a technology which is adaptable to a variable frequency type (tunable).
Hitherto, as a filter which is used as a frequency variable type in a millimeter waveband, (a) a filter using a YIG resonator, (b) a filter with a varactor diode attached to a resonator, and (c) a Fabry-Perot resonator are known.
As the filter using a YIG resonator of (a), a filter which can be used up to about 80 GHz is known in the present situation, and as the filter with a varactor diode attached to a resonator of (b), a filter which can be used up to about 40 GHz is known. Meanwhile, manufacturing gets difficulty at a frequency over 100 GHz.
In contrast, the Fabry-Perot resonator of (c) is well used in an optical field, and a technology which uses the Fabry-Perot resonator for millimeter waves is disclosed in Non-Patent Document 1. Non-Patent Document 1 describes a confocal Fabry-Perot resonator in which a pair of spherical mirrors reflecting millimeter waves are arranged to face each other at the same interval as the radius of curvature, thereby realizing high Q.